1. Field of the Invention
This invention relates generally to a method for converting a lower molecular weight alkane to a higher molecular weight hydrocarbon and more particularly concerns a method for converting a lower molecular weight alkane to synthesis gas and then converting the synthesis gas to higher molecular weight materials in the presence of a Fischer-Tropsch catalyst.
2. Description of the Prior Art
A major source of lower molecular weight alkanes is natural gas. Lower molecular weight alkanes are also present in coal deposits and are formed during numerous mining operations, in various petroleum processes, and in the above- or below-ground gasification or liquefaction of coal, tar sands, oil shale, and biomass.
It is highly desirable to convert lower molecular weight alkanes to more valuable, higher molecular weight materials and a number of attempts to do so have been reported. Because of the high energy costs associated with heating reactants, it is highly desirable that an exothermic process be employed to effect this conversion. One such exothermic process is the exothermic partial oxidation of the lower molecular weight alkane with oxygen to form synthesis gas, which is subsequently converted in a Fischer-Tropsch operation to a higher molecular weight material.
Typical features of many of the reported processes involving the formation of synthesis gas by the partial oxidation of a lower molecular weight alkane and the subsequent conversion of the synthesis gas by a Fischer-Tropsch operation are the use of relatively high pressures in both the synthesis gas formation and conversion steps, the separation of oxygen from air for use in the synthesis gas formation step, and the use of separate heat exchangers, in addition to the synthesis gas formation reactor, to heat the reactants for the synthesis gas formation and to cool the resulting synthesis gas to the temperature range that is suitable for the subsequent synthesis gas conversion reaction.
Furthermore, typically the Fischer-Tropsch conversion of synthesis gas is effected in the absence of an inert diluent such that, as a result of the high partial pressure of synthesis gas and the high rate of synthesis gas conversion, less desirable products such as methane, waxy hydrocarbons and alcohols are formed, and hot spots develop and waxy or carbonaceous deposits are formed in the catalyst bed which reduce catalyst activity. In order to minimize these problems, the synthesis gas conversion is often performed in stages, where only partial conversion occurs in the first stage, and after separation of the products, the remaining unreacted synthesis gas is either converted in a second stage or recycled to the first stage. In either case, high capital costs and operating expenses are involved.
However, such features are not desirable in an economical commercial process. It is highly desirable in a commercial process involving the partial oxidation of a lower molecular weight alkane to form synthesis gas and the subsequent conversion of the synthesis gas by a Fischer-Tropsch operation to effect the synthesis gas formation in a single reactor under at least partially adiabatic conditions such that the exothermic heat generated is utilized in heating the subsequently introduced reactants to the desired reaction temperature. It is also highly desirable to employ air as the source of oxygen in the synthesis gas formation step and to thereby replace the relatively difficult and costly separation of oxygen from air with the relatively simpler and less expensive separation of nitrogen from the final hydrocarbon products and to thereby also utilize the nitrogen component of the air employed in the synthesis gas formation step as an inert diluent in the synthesis gas conversion step to sweep the reactants and products through the reactor, to facilitate the removal of heat generated in the exothermic synthesis gas conversion, and to thereby avoid the development of hot spots and the formation and deposition of waxy or carbonaceous materials in the catalyst bed of the synthesis gas conversion reactor. It is also highly desirable that the Fischer-Tropsch conversion of synthesis gas occur at a high conversion and at a relatively low pressure and with a reduced formation of higher molecular weight hydrocarbon waxes and oxygenates.